Oscillation generator



Feb. 22, 1949. c. w. EARP 2,462,078

030 ILLATION GENERATOR Filed Sept. 10, 1945 2 Sheets-Sheet 1 H6. Hp

YANODE VOLTAGE VOLTAGE SCRE'EN m0 CURRENT Owl/00E 06 7/465 4 InventorCxamaswmmn, Ema

Attorney Feb. 22, 1949. 5. w. EARP 2,462,078

OS 0 ILLATION GENERATOR Filed Sept. 10, 1945 2 Sheets-Sheet 2 2. 0 II is2 l/Lrlwamrom L2 may 3/ 32 usr mm l8 3 v V O 0/9 3 many/5mm? InventorCNHRLES. \MLLmM.EnRP.

Patented Feb. 22, 1949- OSCILLATION GENERATOR Charles William Earp,London, England, assignor,

by mesne assignments, to International Standard Electric Corporation,New York, N. Y., a

corporation of Delaware Application September 10, 1945, Serial No.615,417

In Great Britain September 15, 1944 11 Claims. 1

a The present invention relates to oscillation generators of theelectrical multivibrator type, utilising a single multi-grid electrondischarge device or valve having the electrodes intercoupled to performthe function of the two valves normally present in the well knownorthodox multivibrator circuit.

In the circuits of the type specified the circuit arrangement oscillatesbetween two conditions in which the relative potential difierences oftwo electrodes are reversed the one with respect to the other, thereversal being brought about by the application to an electrode of avoltage of opposite polarity to the biassing voltage already existing onthat electrode. Such an applied voltage may take the form of a voltagepulse generated in another part of the circuit so that the circuitfunctions continuously as an oscillator. Such an oscillator may besynchronised to a train of pulses of suitable voltage polarity appliedto an electrode so as to switch the circuit from one condition to theother, the timing of the synchronising pulse and of the generated pulsebeing suitably chosen.

The single valve type of multivibrator is inherently more stable infrequency than the plural valve version, particularly with regard topower supply fluctuations, but for certain purposes such as forproducing pulses which are employed for opening a gating circuit, forexample, for the selection of a particularly desired pulse train in areceived signal to the exclusion of other pulse trains of the same ordifferent frequencies and interferences, or for frequency dividing, thestability of the single valve type is still not sufiiciently good. It isthe object of this invention to increase the stability of a single Valvecontinuously running multivibrator circuit of the type specified.

This object is attained in accordance with this invention by providing astabilised electric multivibrator comprising a thermionic valve havingin the order named, a cathode, first, second and third grid electrodesand an anode; an operating source therefore having a substantiallyconstant unidirectional potential; first and second resistancesconnecting the positive terminal of the said source to the anode andfirst grid electrodes, respectively; a third resistance connecting thecathode to the negative terminal of the said source; a condenserconnecting the anode to the first grid electrode; means for applying aconstant positive potential to the third grid electrode with respecttothe negative terminal of the said source; a resonant circuit connectedin series with the cathode; and means for deriving output pulses fromthe second grid electrode.

The invention will be described with reference to the accompanyingdrawings, in which:

Fig. 1 shows a schematic circuit diagram of a multivibrator according tothe invention;

Figs. 2 and 3 show waveform diagrams used in explaining the action ofFig. 1; and

Figs. 4 and 5 show respectively two examples of self-gatedmultivibrators according to the invention.

The multivibrator according to the invention shown in Fig. 1 comprises apentode valve l energised by a high tension source intended to beconnected to the terminals 2 and 3, the latter being the negativeterminal which is preferably connected to ground. The anode and screengrid of the valve I are connected to the positive terminal 2 throughresistances 4 and 5 respectively. The cathode is connected to groundthrough a resistance 6 and a parallel resonant circuit consisting of aninductance I and a condenser 8.

A resistance 9 is connected in series with a resistance potentiometer [0between the terminals 2 and 3, and a by-pass condenser ll shunts theresistance 9. The suppressor grid of the valve I is connected directlyto the junction point of the resistances 9 and It. The control grid isconnected to the anode through a condenser l2- and to the moving contactof the potentiometer ill through a resistance l3. It is also connectedto the anode of a diode M, the cathode of which is connected to thejunction point of elements 9 and it. An output terminal I5 is connectedto the terminal I? is connected to the junction point of the elements 6and 1 for applying synchronising pulses. I8 is an additional groundterminal which may be used with IE or ll. A terminal l9 connected to theanode is provided to enable synchronisin'g pulses to be applied, ifdesired, to the anode instead of to the cathode.

The circuit as described without the elements 1 and 8 (the cathode beingconnected directly to- In order to make the operation clear, a specificexample of the circuit of Fig. 1 will be taken, adapted for generatingshort pulses with a repetition frequency of about 100 pulses per second.The circuit elements had the followin values:

H. T. Voltage 200.

It will be noted that the suppressor grid potential ismaintainedconstant at aboutl33 volts and that thecontrolgrid. potential isprevented by; the .diode Hi.- from' exceeding this .value.-:

The action: of thecircuit is rather-complicated. and will. notbe.-described..in.detail. The main features arehowever exhibited in the.curves-of Fig. 2.

The action starts-at zero time, when'the-screen currentis large, andztheanode voltage has its maximumvalue. Since the resistance i is severaltimes larger than the resistance 5, the anode current will be smallcompared with the screen current, and the potential of the cathode willbe determined mainly by the screen current. Thiscathoderpotential.should. beva little higher than the-suppressor gridpotential: which isindicated.

by'; the: :dotted. lineaa in-Fig. 2.. This. :results inthe:anode:current.being; nearly but notquite cut off. At. zero time,the.control.grid.voltageswill be approximately. thesame as :thesuppressor grid voltage; and will subsequently follow the cathodevoltage; owing,to.=the:feed'-back action of the-re.- sistance 6,remaining alwaysslightly lower.

At zero time,. the: anode potential having reached a relatively highvalue, and. the suppressor grid being; only; slightly negative. to thecathode, asmall anode current begins'to flow, with a resulting dropinwanode voltage which is immediately communicated" to-the control gridthrough the condenser 12. The cathode voltage falls with that ofthecontrol grid, and this increases theanode current because thesuppressor grid becomes more positive to the cathode. change is fedround again and the anode voltage falls steeply so that'the cathodevoltage is driven rapidly nearly to zero, and the control grid voltagegoes with it so that the space circuit of-. 'the-.valve--is--nearly butnot quite'cut' off. The

screen. current therefore falls nearly to zero. This generalfallinpotential ofthe valve-electrodes. is. indicated-. by. thepar.ts..b of the three CUIVES.

The; condenser: l2-1 nowcharges; upca slowly through-thez-high;resistance. [3. i11-such;manne1*.: that: theacontrol. grid; potentialrises. slowly. A; similar rise occurs. in .the cathode potential, and.theanodespotential fallsfurther dueito'thesslight The anode. po.tential. fall. is. practically linear: due to the feedeback action,andthis-effect continues'for aarela-.- tivelya'long period, as indicatedby'the. portion 0:

increase in the-anode current.

ofrthe curves. Whenthe: anodepotentialhas.

nearly. reachedthat of the cathode, a short period:

d-IOCCUI'S when the anode current becomes'constant, and thenthe;feed-back;from. the=anode.

The

ceases to operate so that the cathode and control grid potentials bothbegin to rise much more rapidly and in an exponential manner, until thecathode potential reaches the suppressor grid potential, when the anodecurrent is nearly cut off andthe anodepotential. rises sharply at firstand then more slowly and exponentially to the maximum value, when a newcycle commences.

During the period 0 there is a small increase in the screen current, andafterwards a sharp increase to the maximum during the period d.Whiletheanode. voltage is rising exponentially tothe maximumyalue, thescreen current and cathode potentials remain constant owing to theaction of the .diode I4, which prevents the control grid voltage fromrising above the suppressor grid voltage.

The suppressor grid voltage variation will clearly besimilar to thescreen current curve,

I but inverted. Thus short negative pulses with steep, sides will .beobtained for. the terminal IS.

NOW when according to the invention the.reso-.-

nant circuit 1,. 8 is.inc1uded in series with the cathodethe. sharpdownstrokes b of the cathode potential will. excite, theresonant circuitand will cause it to-ring. The naturalfrequency oftheresonant circuitshould preferably be from; five to tentimes the frequency of repetitiondesired for the pulses. An alternating. voltage will be superposed onthe cathodevoltage as shown in Fig. 3. Then the time at which thecathode volt-. age exceeds. the. suppressor gridvoltage at the. endofthe cycleis precisely determined by the steep up-stroke at e: in Fig. 3of the superposed.

2:. alternating voltage wave, andthis accurately fixes the commencementof the new cycle. During the periods of thecathode pulses the resonantcircuit is heavily damped by the low.cathode im-' pedanceof the valve.These pulses should pref--v erably be made of very short duration, bymalt-.- ingthe ratio ofthe resistance of element 4 to thatof element l3small. In the example given. above thisratio is about 0.075. At the endof the. cathode pulse the valve is nearly cut off. so that the dampingissmall andthecircuit continues to ring throughout the. periods between,the oath. ode pulses. As .the cathode current gradually increases, acertain amount otnegative resistance isintroduced, andthe amplitude. ofthe superposed wavestherefore does notnecessarily decreasaas shown atA,.Fig. 3. It may remain sub.- stantially constant, or. it may decreaseand then increase. Examples of these possibilities are. shown in Fig. 3,at B and C.

The time interval between the. cathode. pulses is veryv accuratelydetermined by the resonant circuit i; 8 audit the circuit'constants arechosen. so that a very short cathode pulse is produced a .very stableoscillation is. obtained.

As already explained, the output pulses are preferably derived from thescreen grid, and if very short'pulses' are "required, the pulsesfrom"the" screen grid may "be differentiated in knownmanner; Thepotentiometer IO' provides acone venient fineadjustment of themultivibrator" frequency:

The circuitof Fig.1 may b'eeasil'y synchronised= by a pulse train havingsubstantially the'same' repetition frequency" as that of the circuit;and

this maybe done in a-number of different modes' and by severalmethods'ofsignalinjection. The= synchronising pulses may -be used eitherto ad vance or retard the emission-ofithepulses; at

instants corresponding to either the leading or the, trailing: edges-of.the cathodes. pulses: Thee various possible methods of applying thesynchronising pulses in either sense to any suitable valve electrodewill be understood by those skilled in the art; but the preferred methodis to apply the synchronising pulses in positive sense to terminal lland thence to the cathode. The synchronising pulses do not appreciablyexcite the resonant circuit, and if the repetition frequency of theoscillation when controlled is slightly lower than that of thesynchronising pulse train, synchronisation takes place by the exactdetermination of the leading edges of the cathode pulses, the timingbeing slightly advanced.

This arrangement is found to be quite satisfactory even when thesynchronising pulse train contains interfering pulses of waves so longas they are not strong enough, to be capable of causing the emission ofa cathode pulse too soon by one or more cycles of the superposedalternating current wave. invention to be described presently permitsatisfactory operation even with strong interference. An oscillatoraccording to Fig. 1 may be used to remove moderate interference from aparticular signal pulse train by adding a gating pentode valve to thesuppressor grid to which are applied in positive sense short gatingpulses derived by differentiation from the screen grid of the valve inFig. 1. The signal pulse train is applied to terminal ll of Fig. 1 andalso 'to the control grid of the gating pentode, which will be openedonly at the instants when the signal pulses are due to arrive. In thisway the interference Will be cut off. Preferably the signal pulses areapplied to the gating valve after a slight delay in order to ensure thatthe gate will always open in time. The duration of the gating pulse maybe adjusted by known methods so that the signal pulses will not becurtailed.

Fig. 4-. shows a multivibrator of the kind described with reference toFigs. 1 to 3 to which has been added a self-gating arrangement by meansof which false operation due to strong interference accompanying thesynchronising pulses may be prevented. The multivibrator is shown by theblock 28 which comprises the elements shown in Fig. 1. Only theterminals which are used for connection to the gating circuit are shown,and are given the same designations as in Fig. 1. A gating valve 2| hasits anode connected to the positive high tension terminal 22 through ananode load resistance 23. It will be understood, of course, that acommon high tension supply may be used, if desired, for the valve 2| andfor the multivibrator 20. The screen grid is connected directly to theterminal 22, and the cathods is suitably biassed positively byconnection to the junction point of 'two resistances 24 and 25 connectedin series between terminal 22 and the earth terminal 26. A cathodeby-pass condenser 21 shunts the resistance 25. The synchronising pulsesare applied to the input terminal 28 and thence through a blockingcondenser 29 to the control grid of the valve 2|, the usual gridresistance 38 being provided.

The pulses from the screen grid of the valve of the multivibrator 28(see Fig. 1) are applied through the terminal l5 and through atransformer 3|.to a delay network 32 of any suitable type (such as atransmission line consisting of series inductances and shunt condensers)to the suppressor grid of the valve 2|. A suitable terminatingresistance 33 is provided. Finally, the

Other embodiments of the anode of the valve 2| is connected through ablocking condenser 34 and terminal l9 to the anode of the valve I of themultivibrator 20 (see Fig. 1).

The elements 5 and I6 (Fig. 1) should be chosen to produce suitableshort differentiated pulses which are inverted by the transformer 3|(Fig. 4). Short positive pulses of which the leading edges coincide withthe leading edges of the corresponding negative screen grid pulses willbe obtained, followed by short negative pulses which have no effect. Thepositive pulses are suitably delayed (for example by about 100microseconds) and are applied to the suppressor grid of the valve 2|.

Positive synchronising signals applied at the terminal 28 are amplifiedand inverted by the valve 2| and are applied negatively to the anode ofthe valve I (Fig. 1) about 10 microseconds before the circuit wouldnormally emit a pulse when uncontrolled, thus synchronising themultivibrator. The valve 2| is biassed so that it is normally blocked,but is unblocked by the pulses derived from the multivibrator itself.The delay introduced by the network 32 should be such that when asynchronising pulse is due to arrive, the valve 2| is just unblocked intime by a pulse derived from the preceding synchronising pulse. Thus thevalve (Fig. 1) cannot be afiected by pulses or by any interferingsignals except during the specified 10 microseconds interval. In thisway interfering signals of large amplitude are practically preventedfrom pro ducing false operation and accurate synchronisation by thedesired pulse train is ensured.

The modification of Fig. 4 which is shown in Fig. 5 is adapted forsynchronising the multivibrator to a train of pulses, or for selecting agiven train of pulses from other trains by automatic frequency controlwithout directly controlling the times of emission of the generatedpulses. Those elements of Fig. 5 which are the same as elements of Fig.4 have been given the same designation numbers and will not be againdescribed.

' The transformer 3| and the delay network -32 are not required in thecase of Fig. 5 and have been omitted, the output terminal l5 of themultivibrator 20 being connected directly to the suppressor grid of thevalve 2|. The anode is connected to the terminal 22 through a winding 35which is inductively coupled with the inductance element '1 of themultivibrator 20 as indicated in Fig. 5. Thus the stabilising resonantcircuit is in this case a tuned transformer. It will be understood, ofcourse, that the elements I and 8 may be connected in the same way as inFig. 1. The elements 23 and 34 of Fig. 1 are of course not required inFig. 5 and'have been omitted.

As in the case of Fig. 4, the output pulses from the multivibrator arepreferably differentiated to produce short positive pulses alternatelywith negative pulses which are not used. The positive pulses applied tothe suppressor grid of the valve 2| unblock the valve for theduration ofthe input pulses which are applied at terminal 28 as before, togetherwith any interfering signals which may be mixed therewith. Pulses whichcoincide with the unblocking pulses appear in the anode circuit and areapplied-to the tuned transformer l, 35. According to the manner in whichthe connections to the windings have been poled, these will advance orretard the phase of the ringing of the resonant circuit, sosynchronising the multivibrator to the applied pulse train.

It will be seen that since the unblocking pulses 7 are positive, theywill correspond with the trailing edges of the pulses generated by the:suppressor grid of the valve i in the multivibrator 20, and thereforewith the trailing edges of the cathode pulses. These trailing edgescorrespond in. time with the extreme negative peaks of the ringingvoltage waves of the resonantcircuit "I, 8 By suitable choice of thenatural. period of this circuit, the trailing-edges of the gating pulsescan be made to correspond closely with the initial instants. of zerovoltage of the ringing oscillations,- so that pulsesarriving from thevalve 2! at such instants cause a quadrature drive to the ringing.circuit, and therefore they exert a phase advancing or retardinginfluence. tion frequency of the pulses generated by the multivibratorwhen uncontrolled is supposed to be slightly lower than that of thesynchronising pulse trains, then the transformer 1', 35 should be poledso that when such synchronising pulse rides on the sloping: trailingedge of the corresponding unblock-ing pulse, a quadrature phaseadvancing drive is applied to the ringing circuit and therefore theoscillationperiod will be slightl y shortened to maintain thesynchronism.

If it is: desired to synchronise to a pulse train of slightly lowerrepetition frequency than that of the multivibrator, one of thetransformer windings should be reversed in order that. the

gated pulses may retard the phase of the ringing I! circuit-.-

For actually selecting the desired. pulse train a second gating valve(not shown) is preferably providedwith the. gating pulses applied to thesuppressor grid through a network including a suitable delay so that thegate may be fully opened for the whole periods of the pulses.

What is claimed is:

1. A stabilised electric multivibrator comprising a thermionic valvehaving in the order named,

a cathode, first, second, and third grid electrodes and an anode; anoperating. source therefor having a substantially constantunidirectional potential; first and second resistances connecting thepositive terminal of the said source to the anode and. first gridelectrode respectively; a third resi'stance connecting the cathode tothe negative terminal of the said source; a condenser connecting theanode to the first grid electrode; means for applying. a constantpositive potential to the third: grid electrode with reference to thenegative terminal of the said source; a resonant circuit connectedinseries with the cathode; and means for deriving output pulses from thesecond grid electrode.

2. A multivibrator according to claim 1, in which the resonant circuitcomprises an inductance shunted by a condenser.

3. A multivibrator according to claim 1 in which the resonance frequencyof the resonant circuit is selected to be betweenfive and ten times the:repetition frequency of the pulses generated bythemultivibrator.

If the repeti- 4. A multivibrator according to claim l comprising meansfor preventing the potential of the first grid electrode from becomingpositive with respect to that of the third grid electrode.

5. A multivibrator according to claim 4 in which the said meanscomprises a diode having its anode connected to the first grid electrodeand its cathode connected to the third grid electrode.

6. A multivibrator according to claim 1v com-- prising means forapplying synchronising pulses to the cathode circuit.

'7. A multivibrator according to claim 1 including an inductive windingcoupled to the said inductance, and means for applying synchronisingpulses to the inductive winding.

8. A multivibrator according to claim 1 in cluding means for applyingsynchronising pulses to the anode.

9. A multivibrator according to claim 1 including a gating valve havinga cathode, anode, control grid. and suppressor grid, means for applyinga blocking bias to the said gating valve, means for applying a train ofinput pulses to the control grid of the gating valve, means for applyingoutput pulses from the gating valve to synchronise the multivibrator,and means for applying output pulses from the multivibrator to unblockthe gating valve to enable it to admit each input pulse when it is dueto arrive.

10. A multivibrator according to claim 9 in which. the means forapplying the output pulses from the gating valve comprises means forconheating the anodes of the multivibrator andgat-- ing valves, and in.which the means for applying pulses to unblock the gating valvecomprises a delay network connected between the second grid electrode ofthe multivibrator valve and the suppressor grid of. the gating valve.-

11. A multivibrator according to claim 9 in which the means for applyingthe output pulses from the gating valve comprises means for inductivelycoupling the anode of the gating valve to the said resonant circuit, andin which the means for applying pulses to unblock the gating valvecomprises means for connecting the second grid electrode of themultivibrator valve to the suppressor grid of the gating valve.

CHARLES WILLIAM EARP.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS

